{"id":"src_9bg89t03ak96","title":"A relation between distance and radial velocity among extra-galactic nebulae","authorName":"Edwin Hubble","abstract":"Determinations of the distances of extra-galactic nebulae from Cepheid variables are combined with their radial velocities to demonstrate a linear relationship between distance and velocity. This fundamental observation establishes the expansion of the universe.","tags":["expanding universe","hubble's law","cosmology","redshift","galactic astronomy"],"priceUSDC":"0.0004"}